Wednesday, May 22, 2013

Paper finds a long-term cooling trend in the N. Atlantic and Mediterranean

A paper published in Quaternary Science Reviews finds a long-term cooling trend of the sea surface in the North Atlantic and Mediterranean during the Holocene [past 10,000 years]. According to the authors, "All [7 of] these paleo-temperature records document an apparent long-term cooling during the last 10,000 years." The paper confirms other research finding a cooling of the Sargasso Sea in the N. Atlantic over the past 3,000 years.

All seven temperature proxies from sediments show cooling in the N. Atlantic and Mediterranean over the Holocene. Horizontal axis is years before the present.

Another proxy from the Sargasso Sea, N. Atlantic also shows cooling over the past 3,000 years.

Reconstructions of upper ocean temperature (T) during the Holocene (10–0 ka B.P.) were established using the alkenone method from seven, high accumulation sediment cores raised from the northeast Atlantic and the Mediterranean Sea (36°N–75°N). All these paleo-T records document an apparent long-term cooling during the last 10,000 yrs. In records with indication of a constant trend, the apparent cooling ranges from −0.27 to −0.15°Ckyr−1. Records with indication of time-variable trend show peak-to-peak amplitudes in apparent temperatures of 1.2–2.9°C. A principal component analysis shows that there is one factor which accounts for a very large fraction (67%) of the total variance in the biomarker paleo-T records and which dominates these records over other potential secondary influences. Two possible contributions are (1) a widespread surface cooling, which may be associated with the transition from the Hypsithermal interval (∼9–5.7kaB.P.) to the Neoglaciation (∼5.7–0kaB.P.); and (2) a change in the seasonal timing and/or duration of the growth period of alkenone producers (prymnesiophyte algae). The first contribution is consistent with many climate proxy records from the northeast Atlantic area and with climate model simulations including Milankovitch forcing. The second contribution is consistent with the divergence between biomarker and summer faunal paleo-T from early to late Holocene observed in two cores. Further work is necessary, and in particular the apparent discordance between biomarker and faunal T records for the relative stable Holocene period must be understood, to better constrain the climatic and ecological contributions to the apparent cooling observed in the former records.